This invention relates to an improved combustor for generating a hot gas stream by the combustion of fuel in a pressurized airstream. More specifically, this invention relates to an improved combustor capable of generating a hot gas stream with minimum heat, pressure and velocity losses and with minimum formation of noxious or undesirable exhaust gas constituents.
The present combustor has a wide range of utility, e.g., in room heating and powerhouse steam generating apparatus and is particularly useful in gas turbine engines of both the stationary industrial type and the propulsive type. Although my invention will be described as it is applicable to gas turbine engines for use in automotive and flight propulsion, it is to be recognized that it is equally applicable to any system where fuel is burned in a pressurized airstream to generate a hot gas stream.
In gas turbine engines there are many desiderata for the combustion of fuel for generating a hot gas stream. For example, for efficiency and economy of operation, a hot gas stream should be generated with minimum heat, pressure and velocity losses. Any pressure drop across the combustor lowers the available pressure drop through the turbine and, consequently, the energy available for conversion to useful work in the turbine. For a given fuel flow, this decreases the turbine horsepower and increases the specific fuel consumption Second, it is desirable that the hot gas stream be generated with minimum formation of undesirable exhaust gas constituents, e.g., unburned hydrocarbons, carbon monoxide and oxides of nitrogen. Third, it is desirable that a stable flame of controlled temperature achieving complete combustion of the fuel be maintained within the combustor. In addition, all of these desirable operating characteristics should be obtained over a wide range of ambient pressures and temperatures and at rapid changes in fuel and air mass flow rates through the combustor. Moreover, it is highly desirable that the combustor be compact, light and simple to manufacture and maintain.
Efforts to achieve these desiderata in prior art combustors have not been completely successful. Indeed, these prior art combustors have suffered from one or more serious disadvantages. By way of example, reverse flow-type combustors widely used in aviation and industrial gas turbine engines are wrapped around the outer diameter of the turbine to keep the engine short and are therefore relatively large in diameter. In such combustors the fuel must be injected with a high velocity in order to achieve fast and complete mixing and combustion. This turbulence is generated by a significant pressure drop in the combustor and additional pressure drops result from the large housing and liner surfaces and the turns required to direct air into the inside of the liner. Moreover, the hot and cold air layers are far apart and must be brought together for mixing. Thus, the combustor must be relatively long and the fuel injectors must be placed at the far downstream end thereof. Moreover, the large inner surface absorbs a large amount of heat that must be carried away. And finally, in an effort to reduce weight and cost, these large surfaced combustors are welded from sheet metal, with the result that they are particularly susceptible to failure from vibration, thermal stress and stress concentrations Each of the foregoing is a significant disadvantage.
Accordingly, it is among the principal objects of my invention to provide an improved combustor of compact and simple construction wherein fast and complete mixing of fuel and air and efficient burning is achieved to generate a hot gas stream with minimum heat, pressure and velocity losses.
It is a further object of my invention to provide an improved combustor of relatively short axial length which, nevertheless, provides for relatively long gas flow path lengths and maximum flame stability and burning efficiency with a minimum of heat losses.
It is a still further object of my invention to provide an improved combustor wherein the fuel and pressurized air from the compressor are quickly and completely mixed to provide a substantially homogeneous vapor phase air-fuel premixture which is then thoroughly mixed into a main airstream to produce a combustible air-fuel mixture having a desired air-to-fuel ratio for complete combustion with minimum formation of undesirable exhaust gas constituents.
The present invention is predicated in part upon the concept of providing a combustion chamber in which the flame does not extend in an axial direction, but rather is curved back upon itself in generally the shape of a segment of a toroid. A second important concept of the present invention is to provide a premixing chamber in which an intimate mixture of fuel and air is formed immediately adjacent to the combustion chamber in combination with means for injecting the air-fuel premixture into a main airstream entering the combustion chamber adjacent to the outer periphery thereof with the injected air-fuel premixture having a substantial tangential component such that almost instantaneous, highly localized intermixing of the air-fuel premixture into the main airstream is obtained to generate a combustible air-fuel mixture for burning in the combustion chamber.
In the preferred embodiment of my invention, these and other objects are accomplished by providing a combustor comprising a housing which encloses an arcuate combustor liner effective to constrain the gas flow to form a generally toroidal combustion zone or chamber, an annular air-fuel premixing chamber upstream of, and opening downstream into, the combustion chamber, and a plenum surrounding the premixing chamber for receiving pressurized air from the compressor. The plenum is separated from the air-fuel premixing chamber by a baffle and communicates therewith through a plurality of spaced openings in the baffle angled generally tangentially to the inner surface thereof to provide for the entry of air to generate a vortically flowing, substantially homogeneous vapor phase air-fuel premixture in the premixing chamber. The plenum also communicates with the combustion chamber through an annular orifice outwardly circumferential to the baffle at its downstream end to provide a main airstream entering the combustion chamber. In a preferred form, the main airstream has a vector component of flow effective to generate a helical gas flow path through the toroidal-shaped combustion zone. The vortical discharge from the air-fuel premixing chamber enters the main airstream and becomes thoroughly mixed therein, thereby generating a combustible air-fuel mixture having a predetermined air-to-fuel ratio at the entrance to the combustion chamber for burning therein. The toroidal-helical flow of the combustible air-fuel mixture provides a relatively long gas flow path length in a compact housing. The relationship of the flame which surrounds a vortical recirculation zone of burned gases is effective to cause ignition and efficient and stable burning of the mixture
Other objects and advantages of my invention will be apparent from the following description of a preferred embodiment embodying the present invention and several modifications, reference being had to the accompanying drawings.